As BL Lacs are the most numerous extragalactic \(\gamma \) -ray emitters,
we have cross-matched our catalogue of XMM–Newton BL Lacs with the 4FGL
[1]} to search for \(\gamma \) -ray counterparts. A BL Lac
from our catalogue is considered to be associated with a 4FGL source when their angular distance is equal to or smaller than the semi-major axis of the
95% error ellipse for the position of the 4FGL source.
